The present invention relates to medical radiation therapy and in particular to an electronic computer program for evaluating radiation therapy plans in light of unique patient anatomy and specific plan objectives.
Medical equipment for radiation therapy treats tumorous tissue with high-energy radiation. The amount of radiation and its placement must be accurately controlled to ensure both that the tumor receives sufficient radiation to be eradicated, and that the damage to the surrounding and adjacent non-tumorous tissue is minimized, with the goal of remaining under biological tolerance levels.
In external beam radiation therapy, a radiation source external to the patient treats internal tumors. The external source is normally collimated to direct a beam only to the tumorous site and often modulated with complex dose patterns. The source of high-energy radiation may be x-rays, or electrons from linear accelerators in the range of 2-25 MeV, or gamma rays from highly focused radioisotopes such as a Co60 source having an energy of 1.25 MeV.
Typically, the tumor will be treated from a multitude of geometric directions with the intensity and shape of the beam adjusted appropriately. The purpose of using multiple beams which converge on the site of the tumor is to reduce the dose to areas of surrounding non-tumorous tissue. The angles at which the tumor is irradiated are selected to avoid angles which would result in irradiation of particularly sensitive structures near the tumor site. The angles and intensities of the beams for a particular tumor form a treatment plan for that tumor, with the ultimate goal to generate a three-dimensional (3D) dose that, in sum over all beams, conforms to the target volume(s) and spares healthy tissue
One highly accurate method of controlling the dose to a patient termed intensity modulated radiation therapy (IMRT) employs a radiation source that is segmented with many individual apertures (usually formed by a multi-leaf collimator, or MLC) which in sum produce a complex modulated pattern specific to the anatomy and beam angle. IMRT beams maybe irradiate the patient from many beam angles, or can be delivered while the beam rotates about the patient such as with volume-modulated arc therapy (VMAT) or tomotherapy. U.S. Pat. No. 5,317,616, hereby incorporated by reference, describes the construction of one such machine and one method of calculating the necessary beam intensities as a function of angle.
The control of the radiation therapy machine during a treatment session or multiple treatment sessions is described by a radiation therapy plan developed by one or more healthcare professionals. The treatment plan normally begins a series of treatment goals, for example, defining minimum and maximum dose amounts for particular tissue volumes (called regions of interest, or ROI's), dose homogeneity and the extent to which the dose conforms to the defined ROI. The goals offer leeway in the trade-off between the competing objectives of delivering large doses to diseased tissue while minimizing doses to surrounding healthy tissue. Generally this trade-off exists because of an inability to precisely control radiation falloff at the boundary between healthy and diseased tissue caused by the physics of radiation scattering, and the need for the radiation to pass through healthy tissue on the way to or from the treated tissue.
The goals are used to guide the development of a radiation therapy plan describing, among other parameters, the type of radiation to be used, the orientation of the radiation therapy beams to be directed toward patient at multiple beam stations, the shape for collimation of the beams, and the amount of dose to be delivered at each station. In modern radiation treatment planning, the plan is often generated by the computer via a process called “inverse planning” or dose optimization.
The wide variety of different clinical situations presented by patients (e.g. varying anatomy size, shape, and location) prevents practical reuse of standard radiation plans but instead normally requires a custom radiation therapy plan be developed for each clinical situation. While there are computerized systems that can assist in the development of a clinical radiation therapy plan, the complexity of this process means that clinical radiation therapy plans for most important clinical situations require substantial input from a skilled human planner. Automated systems for developing clinical radiation therapy plans are frequently ineffective when one or more treatment goal is physically unattainable in a particular clinical situation or with a particular radiation therapy machine.
The practical uniqueness of each clinical situation and the wide variation in difficulty presented by each clinical situation make it extremely difficult to assess the quality of a given radiation therapy plan. While periodic contests among radiation therapy planners indicate a significant range in the quality of the plans produced, outside of the contest the individual planner has very little guidance with respect to how well his or her plan achieved the goals relative to other possible plans. See generally, Nelms, Benjamin E. et al., “Variation in external beam treatment plan quality: An inter-institutional study of planners and planning systems”, Practical Radiation Oncology, vol. 2, issue 4, October-December 2012, pgs. 296-305, hereby incorporated by reference.
A simple comparison of a plan against the radiation plan goals on which the clinical radiation therapy plan is founded fails to provide the necessary guidance because failure to meet a radiation plan goal can simply reflect an improperly set plan goal or inherent difficulty in the particular clinical situation (for example, where there is a large overlap between healthy and diseased tissue). Even with all else equal—such as planning equipment, processes, technique, goals, and treatment planner—the degree to which the plan meets all the goals will vary, based solely on the unique anatomical challenges of each patient combined with physical limitations of dose deposition of radiation in the human body.